How can earthquake magnitudes be negative?

Magnitude scales are logarithmic. This means that the amplitude recorded must decrease
10 times for the magnitude to decrease by 1 unit.

At a station close to a magnitude 2 earthquake, the ground moves about 400 micrometres.
So a ground movement of 40 micrometres would signify a magnitude 1 earthquake, 4 micrometres
a magnitude 0 earthquake and 0.4 micrometres a magnitude -1 earthquake.

Earthquakes with magnitude less than 2 are not usually felt and if they are felt then by only a few
people very close to the earthquake. Earthquakes with magnitudes less than 1 are hardly ever felt.
The smallest earthquake felt in the UK had a magnitude of 0.5 and was felt by one person. Earthquakes with negative magnitudes are never felt.